While methods of culture of tobacco differ somewhat in different regions and with various tobacco types, the essential features are much the same.
Seedlings are produced in hotbeds and about 6 to 10 weeks after germination, when they are about 4 to 6 inches in height, the seedlings are transplanted to fields, either by hand or by machine. The distance of planting varies according to the type of tobacco. The rows are generally 3 to 4 feet apart with plants spaced at a distance of 12 to 48 inches in the row. Cigar and burley tobacco are commonly spaced 3 to 31/2 feet by 12 to 27 inches, while fire-cured tobacco is planted in hills which may be as much as four feet apart. Maryland tobacco is transplanted in hills 34 .times. 34 inches or closer, while flue-cured tobacco is spaced 2' .times. 4'.
As a general rule when the plant has attained the desired size, usually at or before attaining flowering, it is debudded or topped; that is, the terminal growth is removed. The number of leaves remaining on each plant commonly varies from 16 to 20 with the cigar, Maryland, burley and flue-cured types and 10 to 14 leaves with fire-cured and dark air-cured tobacco.
Following topping, suckers or lateral shoots develop along the stem and must be removed at more or less frequent intervals, in order to increase leaf development. Topping is generally accomplished by mechanical means which involves removing or cutting away the stem apex, including flowers and some top leaves. This process is usually carried out near the stage of plant maturity, in order to stimulate development of the remaining leaves and to obtain the desirable physical and chemical properties for choice tobaccos. By removing the apex at flowering, the maximum number of harvestable leaves is present and the plant can then divert most of its energy into stimulating the development of these remaining leaves.
When the apex is left intact, most of the plant's energy is diverted to the flowering process, thus preventing expansion of leaves and decreasing yields. However, when the apex is removed, this phenomenon of apical dominance is destroyed with a resulting stimulation of lateral bud or "sucker" growth from the leaf axils. Thus, where only one apex was present, there are now five to ten apices present, which, if left uncontrolled, will further reduce yields. These axillary buds or suckers start to develop soon after the topping operation and prior to the discovery of sucker control agents, these suckers had to be continuously removed by hand to achieve the purpose of topping, since otherwise the suckers themselves would grow at the expense of the development of the desired leaves.
The earliest and perhaps even now, the most common method of controlling the growth of these suckers is to remove them by hand when they are 4 to 6 inches in length. Since the developing suckers are not all in the same stage of growth, the field must be gone over four to six times. It is, therefore, evident that with the increasing cost and decreasing supply of available hand labor, more efficient methods are necessary for control.
The need for commercial chemical tobacco sucker control agents developed as a result of the importance of this cultural practice of topping or removing the apex of the developing tobacco plant when in flower. Certain known chemical products when applied will inhibit the development of suckers or when applied to growing suckers will cause them to wither and inhibit further growth of the suckers. Even now, however, no entirely satisfactory method of sucker control has been developed.
The first attempts made at sucker control by chemical means were through the use of various mineral oil emulsions as well as car and used diesel oils. These compounds are quite injurious to leaf tissue and as a result had to be hand-applied to the cut stalk of each individual plant. Since these materials function mainly through contact activity, volume per plant is critical.
With the discovery and development of maleic hydrazide (MH-30), this compound today is probably the most widely used agent for controlling sucker growth. It is material which is readily translocated within plants and is active in inhibiting cell division in meristematic areas but not cell enlargement. Both timing and environmental conditions are important for maximum effectiveness. If applied too soon after topping, normal leaf expansion will not occur, thus reducing yields. If relative humidity is low or plants are in a wilted state, absorption and effectiveness are greatly reduced. The compound also reduces the specific volume or "filling power" as well as increasing the moisture absorption of the leaf.
The newest agents for sucker control are the fatty alcohols and fatty acid amines. These materials are contact in action; that is, they must come in direct contact with each axillary bud. Control is achieved by selectively killing the primary axillary bud meristem without damaging mature leaf tissue. Since no translocation occurs, subsequent axillary buds will develop into suckers. Therefore, commercial control is only achieved for a matter of 1 to 2 weeks. In practice, these compounds have been shown to injure leaf tissue if recommended application methods are not followed closely. It is also evident that volume per plant is critical to insure complete contact with each leaf axil. Where chemical sucker control agents are used, they are also sometimes unsuitable for the reason that they provide inadequate inhibition of sucker development or cause metabolic changes other than inhibition of suckers, which metabolic changes are undesirable in the tobacco plant. Moreover, there is also the question of the possibility of undesirable residues in the leaf tissue. It is, therefore, an object of this invention to provide new and suitable chemical control of tobacco sucker growth without the disadvantages of prior art chemical treatments.